1. Field of the Invention
The present invention relates to a camera with an AF (Auto Focus) mechanism using a phase-difference method, such as a SRL (Single Reflex Lens) camera. In particular, it relates to an arrangement of AF sensors.
2. Description of the Related Art
An SRL-type camera is equipped with an AF mechanism based on a phase-difference method, which has an optical image-forming system and a sensor module that is constructed from a focus-detection device, such as an IC chip. The optical image-forming system has a separator lens and a condenser lens, whereas the focus-detection device is equipped with a series of line sensors that are arrayed within a projected area of the optical system.
In U.S. Pat. No. 7,493,034, as AF sensor in which a plurality of pairs of line-sensors is arrayed in a projected area is disclosed. An optical system divides a ray bundle from an object into two ray bundles to project a pair of images onto the pairs of line-sensors. Each line-sensor outputs image-pixel signals by photoelectric conversion, and a difference between the positions of the two images, namely a phase difference, is detected. The difference represents an out-of-focus magnitude. Whether or not an object is in focus can be determined by the detected out-of-focus magnitude. When the object in out of focus, an amount of movement by the focusing lens and a shift in its direction are determined. Then, the focusing lens is driven.
Generally, a line sensor is an electric charge storage or accumulation-type sensor, and an accumulation period is adjusted by a monitor sensor that is arrayed alongside the line sensor. For example, as disclosed in U.S. Pat. No. 7,102,675, a monitor sensor consistently detects the intensity of light and outputs monitor signals in order to prevent the target line sensor from receiving a quantity of light exceeding a dynamic range, which would cause electric charges to overflow from the line sensor. The amount of light that each line sensor receives varies according to the brightness distribution of an object. Therefore, a charge-accumulation period is controlled independently for each line sensor. When a monitor signal exceeds a predetermined threshold value, a corresponding line sensor stops the accumulation of electric charges, and accumulated electric charges are temporarily stored in a memory of the line sensor. After the accumulation of electric charges is completed by all of the line sensors, a series of image-pixel signals are output from the line sensors to an AF controller.
Light passing through the optical image-forming system does not sufficiently reach the peripheral region of the projected area due to optical characteristics such ac a decrease in light and vignetting. Therefore, a monitor sensor monitoring a peripheral line sensor positioned near the boundary of the projected area cannot precisely detect the quantity of light reaching the peripheral line sensor.